Titania (i.e., titanium dioxide) nanotubes have shown promise for use in photocatalytic (refs. 1-3), sensor (refs. 4 and 5), biological (ref 6) and other applications. Titania nanotube arrays for such applications have been fabricated by anodization in fluoride-containing media. Zwilling & Darque-Ceretti reported in 1997 that ordered nanoporous structures could be obtained by anodizing titanium in fluoride containing electrolytes (refs. 7 and 8). In 2001, Grimes and co-workers found that titania nanotubes could also be obtained given suitable anodization conditions in hydrofluoric acid (HF) (ref. 9). The need for longer nanotubes led the research groups of Grimes et al. (ref. 10) and of Schmuki et al. (ref. 11) to pioneer the growth of longer titania nanotubes using more alkaline electrolytes with fluoride salts (NaF, KF, NH4F) instead of HF as the fluoride source, and the use of non-aqueous fluoride containing electrolytes (refs. 12-14). In 2005, Nakayama et al. reported that titania nanotubes can be obtained by anodization in a perchloric acid solution (ref. 15). In addition to titania nanotubes produced by anodization, fibrous titanates including TiO2—B nanotubes have been fabricated by a chemical-thermal route, which is typically a variation on the NaOH treatment first used to fabricate nanotubes by Kasuga et al. (ref. 17). Fibrous titanates have found industrial use as a strengthening additive in composite materials (ref. 16).
Previous methods of producing titania nanotubes yield slow growth, and result in nanotubes of limited aspect ratio and structural organization. Anodization of titanium has previously required catalysis by fluoride ions, with no success obtained using other ions for catalysis. There remains a need for improved titanium anodization methods that allow rapid growth and the formation of highly organized arrays of titania nanotubes.